Hydraulic power propel systems in machines are known. Such systems include an engine or another prime mover operating a hydraulic pump. The hydraulic pump provides pressurized fluid to a hydraulic motor, which powers the rotation of wheels, tracks or other ground engaging members of the machine. In typical applications, control of machine motion can be accomplished by using variable displacement hydraulic pumps and/or variable displacement hydraulic motors to adjust the effective speed ratio between the engine and the ground engaging members of the machine.
When machine power is used to propel the machine, for example, when the machine is travelling up an incline or travels on flat terrain, the engine speed and machine ground speed can be controlled by appropriately adjusting the displacement of the hydraulic pump and/or motor to match a desired command signal from the operator. However, when the machine travels down a hill, an over-speed condition may occur. Over-speed is typically used to describe an operating condition in which the machine is pulled along, for example, by force of gravity when travelling down a hill, and in which the engine and/or hydrostatic motor tends to over-speed. The over-speed is caused by the motors acting as pumps to push hydraulic fluid through the hydraulic pump that is connected to the engine and thus tends to increase engine speed by pulling the engine along. Depending on the magnitude of the over-speed condition, the engine and/or hydrostatic motor and pump speeds may increase beyond desired speed limits.
Various solutions have been proposed in the past to address engine and pump over speed in hydraulic power drive systems. One known solution approaches the problem by commanding engine and pump control settings to protect the engine and pump from over-speed when various system parameters, such as engine speed, fluid pressure and the like, indicate that an over-speed condition is present. One such example can be seen in U.S. Pat. No. 5,390,759 (“Gollner”), which issued on Feb. 21, 1995, and is directed to a “Driving Mechanism for an Automotive Propel Drive.” Gollner describes a system in which engine speed and system fluid pressure are used to control the displacement of the pump and/or the displacement of the motors. Such known systems, although at least partially effective in protecting system components from over-speed, are not as effective in providing a retarding effect that can slow the vehicle when travelling down a hill or under other over-speed conditions. Moreover, the effectiveness of such systems greatly depends on the accuracy and quick response time of system sensors monitoring engine speed and fluid system pressure.